Including setup uncertainty in Monte Carlo dose calculation for IMRT

Radiotherapy treatments are planned to deliver a therapeutic dose of radiation to the target while limiting the concomitant dose to normal tissues. But, the targeted area and normal tissues are not stationary within the planned dose distribution due to patient positioning uncertainty prior to each daily fraction. The success of radiation therapy may be compromised by these effects. Inter-fraction setup uncertainty can never be completely eliminated, only reduced through immobilisation techniques. The objective of this work is to include setup uncertainty into clinical Monte Carlo dose calculation for intensity-modulated radiotherapy (IMRT). The specific aims of this project are to incorporate a model of setup uncertainty into clinical Monte Carlo IMRT dose calculations and to compare those results to Monte Carlo dose calculations without the effects of setup uncertainty considered. We have included the setup uncertainty during the "run-time" of simulation by perturbing the incident particles before they enter the patient. Our initial results indicate that in IMRT dose distributions, critical structure dose is usually increased even when the target coverage is similar for a plan that includes setup uncertainty and one that does not. Furthermore, setup uncertainty may actually increase dose homogeneity within the target. Two clinical cases are presented to illustrate our conclusions.